Alkaline cooks, in particular the sulphate process, have conventionally been performed in such a way that the comminuted cellulose fibrous material, commonly wood chips, and the cooking liquor are introduced into a cooking vessel (digester) and, the temperature is then raised to the cooking temperature, typically to 160-180.degree. C., for a given period of time, so that lignin is detached from the wood, resulting in chemical pulp and alkaline waste liquor, called black liquor in the kraft process. In a sulphate process, the cooking liquor is typically white liquor, which is a mixture of NaOH and Na.sub.2 S. The black liquor contains dissolved lignin, salts of organic acids, extractives, and a significant amount of inorganic compounds.
As a rule, the inorganic components are recovered from the black liquor, whereas the organic part is used as combustible matter. For recovery, the black liquor is evaporated to a higher dry matter content (70-90%) in a multi-stage evaporation plant. An evaporation plant operates most commonly in such a way that the vapor having the highest pressure produced in the evaporator is used to heat the evaporator having the next highest pressure. Typically, an evaporation plant comprises 5-7 stages.
A common problem in black liquor evaporation plants is the fouling of heat transfer surfaces, which decreases the efficiency of the heat transfer process. Fouling increases with increasing dry matter content and temperature of the black liquor. In order to make the evaporation plant operate satisfactorily, the fouled surfaces need to be cleaned from time to time.
The worst problems with fouling in black liquor evaporation plants are connected with calcium carbonate scaling. Scales caused by calcium primarily comprise crystalline calcium carbonate scales, or binary salt scales of calcium carbonate and sodium carbonate. Solubility of calcium carbonate is poor. When calcium ions are precipitated as calcium carbonate particles in the black liquor, they remain in the liquid as crystalline particles and thus do not form scale on the heat transfer surfaces.
One of the most significant objectives of the pulp and paper industry in attempting to limit the environmental impact of pulp and paper mills is the minimization of liquid discharges from the mill, in particular from the bleach plant. Commonly referred to as "closing the bleach plant", this objective usually entails some way of collecting and re-using all the liquid effluents from the bleach plant with minimal discharge to the surrounding environment. In the past, this objective was partially achieved in practice by recirculating alkaline bleach plant effluent to the brownstock washing stage and ultimately to the alkaline recovery system.
The recirculation of acidic bleach plant effluents is much more difficult. These acidic effluents, for example, from an acid wash stage (A), an acidic or neutral chelation stage (Q), an acidic ozone stage (Z), or other acidic treatment stage, cannot be returned directly to the conventional recovery system because they contain metals dissolved from pulp.
Finnish patent application 944808 and WO patent application 96/12063 disclose a novel acid treatment in which hexenuronic acid groups are removed from chemical pulp. These acids are removed by adjusting the pH of the pulp to the range of 2-5, preferably 2.5-4, and by adjusting the temperature of the pulp to 90-110.degree. C., and holding the pulp at these conditions for a certain period of time. Also acid soluble metals, e.g. Ca, Mg etc., can be removed efficiently from the pulp by this treatment.
Bleaching effluents, especially acidic effluents, contain undesirable metal ions which can negatively impact the processes and equipment. These metals were previously sewered with acidic bleach plant effluents. In a new closed mill concept it has been proposed that the acidic effluents be evaporated either alone or combined with the alkaline effluents. The preconcentrated effluents can be further evaporated together with black liquor and fed to a recovery boiler. Acidic effluent contains, in particular, a large amount of soluble calcium. Therefore, combining bleaching effluents and filtrates with black liquor may add to the formation of calcium carbonate scale on heat transfer surfaces of a black liquor evaporator which scale negatively impacts the operation of the evaporation plant, as described above. Alkaline effluents may also contain soluble calcium in a detrimental amount if carbonate is not present in a sufficient amount for formation of calcium carbonate.
Co-pending U.S. application Ser. No. 09/026,562 filed Feb. 20, 1998 discloses a method of inhibiting fouling of heat transfer surfaces in a multi-stage alkaline waste liquor evaporation plant by heat-treating a calcium-containing liquor to be treated in the evaporation plant. The calcium-containing liquor is heated to about 110-145.degree. C., and the liquor is maintained at that temperature to reduce the amount of calcium in the liquor by precipitating calcium carbonate, and the heat-treated liquor is evaporated for combustion.
An object of the present invention is to provide a cost-effective and efficient method of inhibiting calcium scaling of the heat transfer surfaces in a black liquor evaporator when bleaching effluent is treated.